Contact for a semiconductor light emitting device

ABSTRACT

A semiconductor structure includes a light emitting layer disposed between an n-type region and a p-type region. A p-electrode is disposed on a portion of the p-type region. The p-electrode includes a reflective first material in direct contact with a first portion of the p-type region and a second material in direct contact with a second portion of the p-type region adjacent to the first portion. The first material and second material are formed in planar layers of substantially the same thickness.

FIELD OF INVENTION

The present invention relates to a reflective contact for a III-nitridelight emitting device.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, composite, or other suitable substrate by metal-organicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), orother epitaxial techniques. The stack often includes one or more n-typelayers doped with, for example, Si, formed over the substrate, one ormore light emitting layers in an active region formed over the n-typelayer or layers, and one or more p-type layers doped with, for example,Mg, formed over the active region. Electrical contacts are formed on then- and p-type regions. III-nitride devices are often formed as invertedor flip chip devices, where both the n- and p-contacts formed on thesame side of the semiconductor structure, and light is extracted fromthe side of the semiconductor structure opposite the contacts.

Silver is often used as a reflective p-contact and is known to besusceptible to transport induced by mechanical stress, chemicalreaction, or electromigration. For example, a III-nitride LED with asilver p-contact is illustrated in FIG. 1 and described in U.S. Pat. No.6,946,685. U.S. Pat. No. 6,946,685 teaches “silver electrodemetallization is subject to electrochemical migration in the presence ofmoisture and an electric field, such as, for example, the fielddeveloped as a result of applying an operating voltage at the contactsof the device. Electrochemical migration of the silver metallization tothe pn junction of the device results in an alternate shunt path acrossthe junction, which degrades efficiency of the device.

FIG. 1 illustrates a light emitting device including a semiconductorstructure that includes a light-emitting active region 130A between ann-type layer 120 of III-V nitride semiconductor and a p-type layer 140of III-V nitride semiconductor. A p-electrode 160 comprising silvermetal is deposited on the p-type layer, and an n-electrode (not shown inFIG. 1) is coupled with the n-type layer. Means are provided by whichelectrical signals can be applied across said electrodes to cause lightemission from the active region, and a migration barrier 175 is providedfor preventing electrochemical migration of silver metal from thep-electrode toward the active region. The migration barrier 175 is aconducting guard sheet. The guard sheet encompasses the silverthoroughly, covering the edges of the silver p-electrode, as illustratedin FIG. 1.

SUMMARY

It is an object of the present invention to include in a p-electrode areflective first material and a second material. In some embodiments,the second material may reduce migration of the first material. Thereflectivity of the contact may be improved over a device with a silvercontact and a guard sheet that encompasses the silver contact.

Embodiments of the invention include a semiconductor structurecomprising a light emitting layer disposed between an n-type region anda p-type region. A p-electrode is disposed on a portion of the p-typeregion. The p-electrode includes a reflective first material in directcontact with a first portion of the p-type region and a second materialin direct contact with a second portion of the p-type region adjacent tothe first portion. The first material and second material are formed inplanar layers of substantially the same thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a light emitting device with a migration barriercovering a silver p-electrode.

FIG. 2 illustrates a portion of a III-nitride device with a silverp-contact patterned with photoresist.

FIG. 3 illustrates the device of FIG. 2 after forming a layer over thepatterned silver p-contact.

FIG. 4 illustrates the device of FIG. 3 after lifting off thephotoresist and forming a guard sheet over the p-electrode.

FIG. 5 illustrates a III-nitride device connected to a mount.

DETAILED DESCRIPTION

In the device illustrated in FIG. 1, in order to seal the silver contactwith a guard sheet, the silver is first etched back from the edge of themesa. The band 10 between the edge of the reflective p-electrode 160 andthe edge of the mesa is referred to as the “black belt” because it isnot as reflective as silver p-electrode 160. The black belt may be, forexample, about 10 microns wide and may represent about 7% of the devicearea. Absorption of light by the black belt may reduce the efficiency ofthe device. In addition, the step 12 created at the edge of silverp-electrode 160 is difficult to seal with guard sheet 175 and thereforeprone to incursion of humidity and out-migration of silver. To minimizethe height of step 12, the silver p-electrode 160 is kept as thin aspossible, for example about 150 nm Stability and reflectivity of thesilver p-electrode may benefit from thicker silver layers, for exampleabout 200 nm.

In embodiments of the invention, after etching back the silverp-contact, the black belt is filled with a metal layer of the samethickness as the silver. The nearly-planar p-contact structure may bemore reflective and better sealed than a conventional contact, such asthe contact illustrated in FIG. 1.

FIGS. 2-4 illustrate forming a reflective contact according toembodiments of the invention. Only a portion of a device is illustratedin FIGS. 2-4. In FIG. 2, a III-nitride semiconductor structure includingan n-type region, a light emitting or active region, and a p-type regionis grown over a growth substrate (not shown), which may be any suitablegrowth substrate and which is typically sapphire or SiC. An n-typeregion 20 is grown first over the substrate. The n-type region mayinclude multiple layers of different compositions and dopantconcentration including, for example, preparation layers such as bufferlayers or nucleation layers, which may be n-type or not intentionallydoped, release layers designed to facilitate later release of the growthsubstrate or thinning of the semiconductor structure after substrateremoval, and n- or even p-type device layers designed for particularoptical or electrical properties desirable for the light emitting regionto efficiently emit light.

A light emitting or active region 22 is grown over the n-type region 20.Examples of suitable light emitting regions include a single thick orthin light emitting layer, or a multiple quantum well light emittingregion including multiple thin or thick quantum well light emittinglayers separated by barrier layers. For example, a multiple quantum welllight emitting region may include multiple light emitting layers, eachwith a thickness of 25 Å or less, separated by barriers, each with athickness of 100 Å or less. In some embodiments, the thickness of eachof the light emitting layers in the device is thicker than 50 Å.

A p-type region 24 is grown over the light emitting region 22. Like then-type region, the p-type region may include multiple layers ofdifferent composition, thickness, and dopant concentration, includinglayers that are not intentionally doped, or n-type layers.

A reflective metal p-contact 26 is formed on p-type region 24.Reflective metal 26 usually includes silver, and may be pure silver, analloy including silver, or one or more silver layers and one or morelayers of a different metal, such as nickel, or other conductivematerial. Reflective metal 26 is between 150 and 250 nm thick in someembodiments. A resist layer 28 is formed over reflective metal 26 andpatterned, then a portion of reflective metal 26, for example in theblack belt region 27, is removed. The portion of reflective metal 26under resist layer 28 remains in the device. By adjusting the etch time,the reflective metal 26 may be removed from under the resist layer 28 upto a distance of a few microns commonly referred to as undercut.

In FIG. 3, resist layer 28 and black belt 27 are covered with a layer 30of approximately the same thickness as reflective metal 26. For example,layer 30 is between 150 and 250 nm thick in some embodiments. Layer 30is selected to be as reflective as possible, without the migrationproblems of silver. Layer 30 may be, for example, a single evaporatedaluminum layer, one or more sputtered aluminum layers, one or morealuminum alloys, an aluminum metal stack such as AlTi, or a non-metallayer such as an Al₂O₃/Al bilayer or a SiO₂/Al bilayer for enhancedreflectivity. The gap between reflective metal 26 and layer 30 may beadjusted from zero to less than two microns by controlling the undercutof the reflective metal 26 and selecting an appropriate depositiontechnique of layer 30.

The resist layer 28 is then lifted off, exposing reflective metal 26 andleaving behind layer 30 in the black belt 27. In FIG. 4, a guard sheet32 is formed over the p-electrode, which includes reflective metal 26and layer 30. Guard sheet 32 may be, for example, one or more metalssuch as titanium, tungsten, or one or more alloys, or one or moredielectrics for improved reflectivity, such as SiN_(x), SiO_(x), orAl₂O₃. In some embodiments, guard sheet 32 is a layer of TiWN sandwichedbetween two layers of TiW. In some embodiments, layer 30 is AlTi andguard sheet 32 includes at least one layer of TiW. AlTi may provideenhanced adhesion to a TiW guard sheet layer. In some embodiments, theguard sheet includes an underlayer and/or overlayer such as nickel forimproved adhesion.

FIG. 5 illustrates an LED 42 connected to a mount 40. Before or afterforming the above-described p-electrode on p-type region 24, portions ofan n-type region are exposed by etching away portions of the p-typeregion and the light emitting region. The semiconductor structure,including n-type region 20, light emitting region 22, and p-type region24 is represented by structure 44 in FIG. 3. N-contacts 46 are formed onthe exposed portions of the n-type region.

LED 42 is bonded to mount 40 by n- and p-interconnects 56 and 58.Interconnects 56 and 58 may be any suitable material, such as solder orother metals, and may include multiple layers of materials. In someembodiments, interconnects include at least one gold layer and the bondbetween LED 42 and mount 40 is formed by ultrasonic bonding.

During ultrasonic bonding, the LED die 42 is positioned on a mount 40. Abond head is positioned on the top surface of the LED die, often the topsurface of a sapphire growth substrate in the case of a III-nitridedevice grown on sapphire. The bond head is connected to an ultrasonictransducer. The ultrasonic transducer may be, for example, a stack oflead zirconate titanate (PZT) layers. When a voltage is applied to thetransducer at a frequency that causes the system to resonateharmonically (often a frequency on the order of tens or hundreds ofkHz), the transducer begins to vibrate, which in turn causes the bondhead and the LED die to vibrate, often at an amplitude on the order ofmicrons. The vibration causes atoms in the metal lattice of a structureon the LED 42 to interdiffuse with a structure on mount 40, resulting ina metallurgically continuous joint. Heat and/or pressure may be addedduring bonding.

After bonding LED die 42 to mount 40, all or part of the substrate onwhich the semiconductor layers were grown may be removed by anytechnique suitable to the particular growth substrate removed. Forexample, a sapphire substrate may be removed by laser lift off. Afterremoving all or part of the growth substrate, the remainingsemiconductor structure may be thinned, for example byphotoelectrochemical etching, and/or the surface may be roughened orpatterned, for example with a photonic crystal structure. A lens,wavelength converting material, or other structure known in the art maybe disposed over LED 42 after substrate removal.

Embodiments described above may have several advantages over thestructure illustrated in FIG. 1. The p-electrode structure in theembodiments above may be more planar, thereby reducing stressconcentration points and improving the integrity of the guard sheet byeliminating the need for the guard sheet to cover a step. The reflectivemetal may be made thicker without enhancing the problems related tocovering a step at the edge of the reflective metal with a guard sheet.Optical losses from the chip may be reduced by reducing the amount oflight absorbed by the black belt. Problems associated with silverpeeling off the underlying semiconductor material may be reduced, aslayer 30 may protect the edges of reflective metal 26 during subsequentprocessing. An aluminum layer 30 may serve as a sacrificial anode, whichmay inhibit or delay silver electro-corrosion. Silver migration in theblack belt may be reduced by the high electrical conductivity and lowerelectric field of an aluminum layer 30.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

1-6. (canceled)
 7. A method comprising: growing a semiconductorstructure comprising a light emitting layer disposed between an n-typeregion and a p-type region; forming a reflective first material on thep-type region; forming a resist layer on the reflective first material;patterning the resist layer to form an opening in the resist layer;removing a portion of the reflective first material corresponding to theopening in the resist layer; forming a second material on a remainingportion of the resist layer and a portion of the p-type region exposedby removing a portion of the reflective first material; and removing theremaining portion of the resist layer.
 8. The method of claim 7 whereinthe first material and second material are substantially the samethickness.
 9. The method of claim 7 wherein the first material comprisessilver.
 10. The method of claim 7 wherein the second material comprisesaluminum.
 11. The method of claim 7 further comprising forming a thirdmaterial on the first and second material, wherein the third material isconfigured to prevent migration of the first material.
 12. The method ofclaim 11 wherein the third material comprises titanium and tungsten.